Public awareness of the dangers posed by carbon monoxide in the home and continuing advances in carbon monoxide detection technology and reliability has greatly increased the number of carbon monoxide detectors installed in homes. Unlike hazardous gas detectors for industrial applications that are serviced and calibrated regularly, carbon monoxide detectors for use in domestic applications are intended to be “fit and forget” devices. This places a major demand on the sensor used in a domestic gas detector as it must maintain its performance over periods that run into years, all without attention from the owner.
However, with the very long life times expected for a domestic carbon monoxide detector, possibly extending to five years or more, some degradation and loss of accuracy must be expected. Indeed, there are three main potential causes for loss of performance of the sensors typically used in domestic carbon monoxide detectors. First, the active part of the sensor, the membrane electrode assembly or “MEA,” can be damaged by either loss of hydration or chemical poisoning. The effect of this is to reduce the output of the sensor for a particular level of carbon monoxide. As a result, the exposure times or concentration levels before an alarm will be sounded are increased. Second, the electrical contacts to the MEA can be lost. This results in a complete loss or intermittent output from the sensor in the presence of carbon monoxide, which results in no or intermittent alarm output in the presence of carbon monoxide. Third, the gas diffusion aperture in the top of the sensor could become blocked. This results in zero output from the sensor when carbon monoxide is in the environment because none of this carbon monoxide can reach the MEA. With each of these performance affecting events, the result is that the carbon monoxide alarm no longer provides adequate warning during periods of high carbon monoxide concentrations or long carbon monoxide exposures.
Recognizing the serious dangers posed by such a degradation in performance, some modern domestic carbon monoxide detectors employ a mechanism to apply hydrogen as a calibration gas to the top of the sensor. Indeed hydrogen is used as a calibration gas for all types of sensors that detect reducing gases, including carbon monoxide detectors. This is because electrochemical carbon monoxide sensors are capable of sensing a number of gases in addition to carbon monoxide. These gases include hydrogen, sulfur dioxide, and nitrogen oxides. Hydrogen is used as the calibration gas because it is easily produced, and is relatively non-toxic and safe, provided that the concentration is kept below explosive levels.
Unfortunately, while typical carbon monoxide sensors do react to hydrogen, the behavior of the sensor towards hydrogen is not the same as its behavior towards carbon monoxide. Specifically, under conditions of chemical poisoning of the MEA the sensitivity to hydrogen can actually increase while the sensitivity towards carbon monoxide decreases. In such a condition, utilization of hydrogen as the calibration gas will provide a false level of security that the carbon monoxide detector is operating properly when, in fact, its ability to sense carbon monoxide has been substantially impaired.
There exists, therefore, a need in the art for a domestic carbon monoxide sensor that provides reliable self calibration and testing of its ability to properly detect concentrations of carbon monoxide under various failure conditions, including poisoning of the MEA.